1. Field of the Invention
The present invention relates to an electrostatic lens for focusing the beams of charged particles, more particularly of ions, which have at least two electrodes being designed as an electric conductor with a ring-shaped section, the inner edge of which is essentially circular, whereas each electrode is connected to a power supply, can be connected to an electric potential and is fastened on a holding device.
2. Description of the Prior Art
Electrostatic lenses are used in particle optical systems and may be used for multi-various applications. Of particular interest is the application of ion beam lithography used in structure semiconductors. In the case of ion beam lithography, an ion beam pictures the structures of a shadow mask on a wafer in a usually reduced picture. In order to produce a well defined picture of the mask structures on the wafer with the help of the ion beam, the ion beam is led through an imaging system in which a number of electrodes held at different electrostatic potentials are combined into electrostatic lenses.
Due to the extraordinarily high requirements expected in the production of semiconductors, the electrostatic field in ion beam lithography has to be very accurately defined in the areas traversed by the ion beam. The efficiency of such a physical system is thus essentially dependent on the fact that the actual electrostatic fields are corresponding with the set points. This means that on one hand, the shape of the electrodes--which is usually radial symmetric--and their spacing is "perfect" and, on the other hand, that all the parts of the projection system have to be "perfectly" aligned. The production of such a "perfect" system is implying considerable measures and costs. Furthermore, in case of a defect or a change in shape or alignment of the electrodes, for example, due to environmental factors, the repair or realignment of the system is particularly difficult.
If, during the mounting of an ion beam lithography installation, inaccuracies in production or misalignments are making their appearance, the image will be faulty also. Thanks to electric or magnetic multipole fields, the correction of image distortions is possible and well known to those skilled in the art.
In U.S. Pat. No. 2,919,381, multipole electrodes are used to correct image distortions. Such a multipole electrode is divided into sectors functioning as partial electrodes and having their own power supply. In feeding potential differences between the partial electrodes of a multipole electrode, a multipole field is being produced that corrects partially the different image distortions.
U.S. Pat. No. 4,963,748 teaches combining multipole electrodes longitudinally, thus further improving the image accuracy, as well as electrodes having a rotationally symmetrical basic shape and presenting, among others, advantages in manufacturing techniques.
In a multipole electrode of the type mentioned above, each partial electrode has to be connected individually to a supply point and has to be triggered by it. This may imply considerable measures. Moreover, the potential difference between adjacent partial electrodes may lead to big field intensities which are occurring particularly on the edges of the partial electrode of an essentially rotationally symmetrical electrode.